Autofocus feedback positioning system for laser processing

Abstract

The present invention provides a laser processing and autofocusing system that measures the position of the work-piece at the machining spot, to allow the laser processing of work-pieces which are not flat and have surface variations, with the autofocusing system being able to compensate for these variations. During laser machining, it is desirable to keep the laser focus at the same height with respect to the surface of the sample for the best possible machining quality. Not all samples are flat, though, and it thus becomes necessary to map the surface of the sample accurately and to correct the focusing of the laser beam on the fly. The system includes a processing laser beam directed onto the surface at normal incidence and a light line projected onto the surface in the vicinity of the area being machined but at an angle with respect to the surface normal. The focusing optics for the machining laser and light line generator are fixed relative to each other on a positioning stage. If the work-piece surface deflects up or down, the light line moves correspondingly in the horizontal plane with respect to the laser focal spot. This change is monitored on the imaging system and the change of position is used to drive a controller. A proportional voltage set-point is produced by the electronics for feedback to the controller which uses this voltage reference to move the positioning stage to correct accordingly so the focal point of the processing laser beam is once again positioned at the surface of the work-piece.

Therefore what is claimed is:  
     
       1. An autofocus feedback system for a laser processing system, comprising: 
       a focusing and directing system disposed to receive a first laser beam for processing a work-piece and focus and direct said first laser beam onto a surface of the work-piece, an actuator connected to said focusing and directing system for controlling movement of the focusing and directing system for adjusting where the focused laser beam hits on the surface of the work-piece and for controlling a distance of a focal spot of said laser beam from the surface;  
       a light source and a light shaping and projecting system disposed to receive light from said light source and project a selected patterned beam of light onto a surface of said workpiece in a region of the surface being processed with said first laser beam, said light shaping and projecting system producing a light pattern including an elongate portion, said focusing and directing system and said light shaping and projecting system being disposed with respect to each other so that the focused first laser beam incident on the surface and the patterned beam of light incident on the surface are at a pre-selected angle with respect to each other;  
       image detection means for receiving an image of said patterned beam of light reflected from said surface; and  
       processing means for calculating a shift in position of said surface in a vicinity of the region of the surface being laser processed from said image of said patterned beam of light reflected from said surface, said processing means being connected to said actuator so that as the position of said surface shifts, said actuator moves said directing and focusing system to return said focal spot to a pre-selected distance from said surface.  
     
     
       2. The autofocus system according to  claim 1  wherein said directing and focusing system directs said focused beam onto the surface at a direction which is substantially normal to the surface of the work-piece. 
     
     
       3. The autofocus system according to  claim 1  wherein said pre-selected distance of the focal spot from the surface is zero so that said focal point is coincident with said surface. 
     
     
       4. The autofocus system according to  claim 1  wherein said image detection means is connected to an image display means for displaying an image of said pattern of light reflected from said surface. 
     
     
       5. The autofocus system according to  claim 1  wherein said pattern of light produced by said light shaping and projecting system is a straight light line. 
     
     
       6. The autofocus system according to  claim 5  wherein said directing and focusing system and said light shaping and projecting system are disposed with respect to each other so that said light line is projected onto said surface in such a way as to intersect a region where said laser beam strikes the surface of the work-piece. 
     
     
       7. The autofocus system according to  claim 1  wherein said light source is a second laser emitting a laser beam having a wavelength different from said laser beam produced by said first laser. 
     
     
       8. The autofocus system according to  claim 1  including filter means located in front of said detection means for filtering light entering said detection means to pass light of wavelengths corresponding to said reflected pattern of line. 
     
     
       9. The autofocus system according to  claim 1  wherein said detection means is an imaging detector. 
     
     
       10. The autofocus system according to  claim 9  wherein said imaging detector is a CCD camera. 
     
     
       11. The autofocus system according to  claim 7  wherein said second laser is a helium neon laser beam emitting at a wavelength in the red. 
     
     
       12. The autofocus system according to  claim 7  wherein said second laser is a diode laser. 
     
     
       13. The autofocus system according to claims  1  wherein said pre-selected angle is between about 25 to 75 degrees from a centerline of said machining laser beam. 
     
     
       14. The autofocus system according to  claim 8  wherein said filter means is a bandpass filter which passes light having a wavelength corresponding to said light source. 
     
     
       15. The autofocus system according to  claim 1  wherein said focusing and directing system and said light source and said light shaping and projecting system are mounted on a positioning stage with the positioning stage disposed so said focusing and directing system receives said machining laser beam and focuses and directs it to said work-piece, and wherein said actuator is connected to said positioning stage. 
     
     
       16. The autofocus system according to  claim 15  wherein said positioning stage includes adjustment means for adjusting the angle between the patterned beam of light incident on the surface and the machining laser incident on the surface. 
     
     
       17. A laser processing system with autofocus feedback, comprising: 
       a first laser for processing a work-piece, a focusing and directing system disposed to receive a first laser beam from said first laser and focus and direct said first laser beam onto a surface of the work-piece, an actuator connected to said focusing and directing system for controlling movement of the focusing and directing system for adjusting where the focused laser beam hits on the surface of the work-piece and for controlling a distance of a focal spot of said laser beam from the surface;  
       a light source and a light shaping and projecting system disposed to receive light from said light source and project a selected patterned beam of light onto a surface of said work-piece in a region of the surface being processed with said first laser beam, said light shaping and projecting system producing a light pattern including an elongate portion, said focusing and directing system and said light shaping and projecting system being disposed with respect to each other so that the focused first laser beam incident on the surface and the patterned beam of light incident on the surface are at a pre-selected angle with respect to each other;  
       image detection means for receiving an image of said patterned beam of light reflected from said surface; and  
       processing means for calculating a shift in position of said surface in a vicinity of the region of the surface being laser processed from said image of said patterned beam of light reflected from said surface, said processing means being connected to said actuator so that as the position of said surface shifts, said actuator moves said directing and focusing system to return said focal spot to a pre-selected distance from said surface.  
     
     
       18. The system according to  claim 17  wherein said directing and focusing system directs said focused beam onto the surface at a direction which is substantially normal to the surface of the work-piece. 
     
     
       19. The system according to  claim 18  wherein said pre-selected distance of the focal spot from the surface is zero so that said focal point is coincident with said surface. 
     
     
       20. The system according to  claim 18  wherein said image detection means is connected to an image display means for visually displaying an image of said pattern of light reflected from said surface. 
     
     
       21. The system according to  claim 18  wherein said pattern of light produced by said light shaping and projecting system is a straight light line. 
     
     
       22. The system according to  claim 21  wherein said directing and focusing system and said light shaping and projecting system are disposed with respect to each other so that said light line is projected onto said surface in such a way as to intersect a region where said laser beam strikes the surface of the work-piece. 
     
     
       23. The system according to  claim 18  wherein said light source is a second laser emitting a laser beam having a wavelength different from said laser beam produced by said first laser. 
     
     
       24. The system according to  claim 18  including filter means located in front of said detection means for filtering light entering said detection means to pass light of wavelengths corresponding to said reflected pattern of line. 
     
     
       25. The system according to  claim 18  wherein said detection means is an imaging detector. 
     
     
       26. The system according to  claim 25  wherein said imaging detector is a CCD camera. 
     
     
       27. The system according to  claim 18  wherein said second laser is a helium neon laser beam emitting at a wavelength in the red. 
     
     
       28. The system according to  claim 17  including an X-Y translation stage wherein said work-piece is secured to said X-Y translation stage while being laser processed. 
     
     
       29. A method for autofocus feedback for maintaining a focal spot of a processing laser at a pre-selected distance from a surface of a work-piece being laser processed, said pre-selected distance including said focal spot being coincident with said surface or at a specified distance from said surface, the method comprising the steps of: 
       focusing a processing laser beam to a focal spot and directing said focused processing laser beam onto a surface of a work-piece, projecting a patterned beam of light onto a surface of said work-piece in a region of the surface being processed at a pre-selected angle with respect to said focused processing laser beam incident on the surface, said patterned light beam including an elongate portion; and  
       capturing an image of the patterned light beam reflected from the surface and calculating a shift in position of said surface in a vicinity of the region of the surface being laser processed from said image of said patterned beam of light reflected from said surface and if said surface has shifted adjusting said focused processing laser beam to return said focal spot to said pre-selected distance from said surface.  
     
     
       30. The method according to  claim 29  wherein said patterned light beam is a straight light line. 
     
     
       31. The method according to  claim 29  wherein said focused processing laser beam is directed onto the surface of the work-piece at normal incidence to said surface. 
     
     
       32. The method according to  claim 29  wherein if said surface has shifted said captured reflected patterned light beam shifts and said step of adjusting said focused processing laser beam to return said focal spot to said pre-selected distance from said surface includes producing a signal proportional to a distance said patterned light beam has shifted and using said signal as a feedback signal to drive a positioning means on which is mounted a processing laser focusing and directing system to re-position said focal point of said processing laser beam back to said pre-selected distance from said surface of the work-piece.